In many IR-cameras a scanning element is arranged in front of an IR-detector, said element consisting of rotating prisms of e.g. silicon. Scanning elements are also used which contain tilting mirrors and rotating, reflecting drums. Such a scanning element causes the scanned object to be scanned in two directions.
Such scanning elements are operative so that the IR-detector, beside receiving radiation from the scanned object, also receives radiation from the camera housing and other parts included in the scanning device.
Owing to the manner of action of the scanning element only a certain portion of a sweep in the line and frame part can be used as a video signal. This part is approximately 70% of the sweep. The remaining part (30% or more) of the sweep is not pure image information but contains a signal from the scanning element itself.
A standard type of IR-camera is designed so that the detector signal is d.c. voltage-coupled to the image tube. This is often a good solution, but in certain applications, where the temperature level of the object is of no interest, it is nevertheless a disadvantage to always have to balance out manually the detector signal to zero (grey image) in order to obtain a true grey-tone image. The balancing depends upon the absolute temperature of the object concerned, and if, for example, in the first place the grey-tone image is balanced with an IR-camera directed towards a certain object for the study of the same, and subsequently the camera is directed towards another object at a somewhat different temperature level, a new balancing will have to be carried out.
To avoid this manual balancing, it is appropriate to a.c. voltage-couple the detector signal to the image tube. According to the known technique, the procedure is usually to insert a capacitor in the amplifier chain in series with the amplifier. On the output side of the capacitor the video signal mean value is then assumed to be zero, possibly with a constant voltage added from a separate adjusting element so as to adapt the d.c. voltage level of the signal to the grey level of the cathode-ray tube included in the image representation unit. As mentioned earlier, it is only a certain portion of the video signal which contains the desired information. It is not desirable that the portion of the video signal which does not contain valuable information should influence the mean-value formation in the a.c. voltage coupling in the system, since the automatic balancing would then be incorrect.